This grant proposes preclinical studies to improve the utility and efficacy of inhaled nitric oxide (NO). This laboratory has reported pioneering studies providing evidence that inhaled NO gas produces selective Pulmonary vasodilation. Inhaling low levels of NO has been shown to selectively reverse pulmonary vasoconstriction due to hypoxia and other stimuli and increase systemic oxygenation by selectively vasodilating ventilated lung regions in patients and animal models of acute lung injury. Since1999, over 120,000 patients per year have breathed low levels of NO to treat pulmonary hypertension and acute respiratory failure. Unfortunately, approximately 40% of patients do not respond to NO inhalation with pulmonary vasodilation and improved oxygenation. The etiology of this variable response remains incompletely understood. Studies supported by this grant will have four aims: (1) identify mediators of the impaired hypoxic pulmonary vasoconstriction (HPV) due to endotoxin-induced lung injury, (2) elucidate the roles of NO, leukotrienes, and reactive oxygen species in the impairment of HPV associated with ventilator-induced lung injury (VILI), (3) examine the role of leukotrienes in the impairment of pulmonary vascular responsiveness to inhaled NO in acute lung injury, and (4) evaluate the impact of inhibitors of leukotriene signaling, as well as nitric oxide inhalation, on the development of VILI. Studies proposed in the first three acute lung injury due to endotoxemia and high tidal volume ventilation. New therapeutic approaches developed as a result of studies in mice will be tested in a second species, the lamb, as a key next step before bringing these new strategies to the patient bedside. By understanding the nature of the interaction of endogenous and inhaled NO and leukotriene synthesis in the pulmonary vascular dysfunction associated with acute lung injury, the proposed studies should enhance the utility of inhaled NO therapy in acute lung injury.